The sites and mechanisms of synthesis of the endogenous opioid peptides and the relationship that exist among them are investigated in rats. Developmental immunohistochemistry and tritium labeled thymidine autoradiography are used to establish the time of origin of each opioid-neuronal system, detect beta-endorphin in alpha-2 neurons of immature brain, locate common precursor mechanisms for leu-enkephalin in the striatopallidal and dynorphin in the hypothalamic neuronal systems, and determine whether dynorphin and vasopressin have two separate precursor mechanisms.